Here's why:

* Planet Mercury: This planet was named after the Roman messenger god Mercury, and it was observed and recorded by ancient civilizations long before we understood the nature of elements.

* Element Mercury: The element mercury was discovered much later, and its name was derived from the planet, not the other way around. Alchemists named the element after the planet because they associated it with the planet's quick, silvery nature.

So, the planet Mercury existed in the solar system long before we discovered the element mercury.